Joint pathology is a debilitating disease that can have devastating effects on an individual's quality of life and physical well being. The incidence of joint pathology is quite high, with an individual's chance of acquiring joint pathology increasing with age.
Perhaps the most common joint pathology is arthritis, which is the second most common chronic condition in the United States, affecting over 46 million people annually. There are many methods of treating or temporarily alleviating arthritis, although in severe cases the only way to stop pain and regain lost motion is through total joint replacement.
Total joint replacement is thus the standard treatment for serious joint pathology, and over 980,000 joint replacement procedures were performed in the United States in 2005. Hip and knee arthroplasties were first performed in the 1960's, and such arthroplasties are generally 90-98% successful over a 10 year span. However, ankle arthroplasty is typically much less successful.
A prominent weakness of replacement (artificial) ankle joints is the bond between the talar component of the prosthesis and the talus. Early artificial joints were cemented into place. Over time, it was recognized that if certain materials (such as titanium) were used in fabricating the artificial joint, bone would grow around and into the surfaces of the artificial joint components that were in contact with the residual (remaining) bone, increasing the likelihood that the artificial joint would be successful over the long term. This process of bone growth is referred to as osteointegration. Theoretically, osteointegration can provide the required bond between an artificial joint component and the bone. If too much strain (motion) occurs between the artificial joint component and the bone, osteointegration will be unlikely to occur. Artificial ankle joints appear particularly at risk for such failure because their smaller relative size (and resulting contact area) increases the stress at the joint's bone-implant interface when a patient stands or walks. This increase in stress magnitude would in turn produce higher strains, which may subsequently reduce the likelihood of osteointegration.
Conventionally, a complicated weight-bearing rehabilitation protocol is used to allow patients to gradually adjust (increase) the forces applied to a new prosthesis, allowing the bone to take hold. However, osteointegration rates can vary between patients, and such a rehabilitation procedure does not actually measure the degree of osteointegration for a particular patient. Unfortunately, conventional techniques cannot be used to non-invasively determine when sufficient osteointegration has occurred to enable normal stress loads to safely be applied to such artificial ankle joints.